There are videos on the Internet showing that J305 tubes are very sensitive to UV rays. They literally go crazy under the influence of this type of radiation.
We conducted our own quick UV test of Geiger tubes J305 , which are supplied as an option with the Geiger counter module GGreg20_V3, and filmed this video: .
As a result of the test, we determined that the J305 tube from our batch is not sensitive to UV 395nm (the flashlight we had on hand). At least, we were not able to reproduce/observe such sensitivity as in the video from the Internet.